Make DependentHandle public (#54246)

* Move DependentHandle to System.Runtime

* Update DependentHandle APIs to follow review

* Make DependentHandle type public

* Update DependentHandle on Mono runtime

* Add allocation checks to DependentHandle APIs

This avoids throwing ExecutionEngineException-s if one of the public APIs is called on a non-allocated DependentHandle instance

* Add more unit tests for new public DependentHandle APIs

* Add faster, unsafe internal APIs versions to DependentHandle

* Naming improvements to Ephemeron type

The ephemeron type is checked in the Mono runtime in "object.c" as follows:

m_class_get_image (klass) == mono_defaults.corlib && !strcmp ("Ephemeron", m_class_get_name (klass))

As such, the namespace it belongs to in the managed runtime doesn't matter: the VM will just check
that the type name matches, and that the type is in fact defined in corelib. This means we can
just move it to System.Runtime without worrying about it being properly managed in the VM.
Additionally, the type is defined in "sgen-mono.c" as follows:

typedef struct {
    GCObject* key;
    GCObject* value;
} Ephemeron;

So as long as the layout matches the one of the type defined in C# (which it does), we're also free
to rename the fields to better follow the naming guidelines, and the VM will have no issues with it.

* Code style tweaks, improved nullability annotations

* Remove incorrect DependentHandle comment on Mono

* Add default Dispose test for DependentHandle
Co-authored-by: NStephen Toub <stoub@microsoft.com>

* Fix race condition in DependentHandle on Mono

* Optimize DependentHandle.nGetPrimary on CoreCLR

Removed internal call, same optimization as GCHandle

* Small IL codegen improvement in DependentHandle.nGetPrimary

* Simplify comments, add #ifdef for using directive

* Minor code style tweaks

* Change nGetPrimaryAndSecondary to nGetSecondary

* Minor code refactoring to DependentHandle on Mono

* Rename DependentHandle FCalls

* Remove DependentHandle.UnsafeGetTargetAndDependent

* Remove DependentHandle.GetTargetAndDependent

* Fix FCall path for internal DependentHandle APIs

* Add more DependentHandle unit tests

* Reintroduce DependentHandle.GetTargetAndDependent()

This fixes a bug due to a race condition in ConditionalWeakTable<K, V>, which relies on this method which atomically retrieves both target and dependent with respect to target being set to null concurrently by other threads. This also exposes the same API publically to allow consumers to potentially implement custom conditional weak tables in the same manner.

* Minor IL tweaks to produce smaller IR in the JIT

* Add DependentHandle.StopTracking() API

This also fixes two potential GC holes when setting DependentHandle.Target (see conversation from https://github.com/dotnet/runtime/pull/54246#issuecomment-863285327 onwards)

* Rename InternalSetTarget to StopTracking, remove redundant param

* Remove FCUnique from InternalStopTracking

This was added in https://github.com/dotnet/runtime/pull/39810 to avoid a collision with MarshalNative::GCHandleInternalSet, as the two FCalls had identical implementations and their entry points were not unique. This should no longer be needed after 099fc478551f46cc54e7a18a32d9a9ac73727c73, as that changed both the signature and the implementation of this FCall.

* Update API surface to match approved specs from API review

* Update DependentHandle XML docs
Co-authored-by: NStephen Toub <stoub@microsoft.com>

src/coreclr/System.Private.CoreLib/System.Private.CoreLib.csproj

@@ -206,13 +206,13 @@
     <Compile Include="$(BclSourcesRoot)\System\Reflection\Metadata\RuntimeTypeMetadataUpdateHandler.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Resources\ManifestBasedResourceGroveler.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\CrossLoaderAllocatorHashHelpers.cs" />
-    <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\DependentHandle.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\GCHeapHash.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\CastHelpers.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\ICastableHelpers.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\RuntimeFeature.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\RuntimeHelpers.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\TypeDependencyAttribute.cs" />
+    <Compile Include="$(BclSourcesRoot)\System\Runtime\DependentHandle.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\GCSettings.CoreCLR.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\InteropServices\ComTypes\IEnumerable.cs" />
     <Compile Include="$(BclSourcesRoot)\System\Runtime\InteropServices\ComTypes\IEnumerator.cs" />

src/coreclr/System.Private.CoreLib/src/System/Runtime/CompilerServices/DependentHandle.cs

-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-
-namespace System.Runtime.CompilerServices
-{
-    // =========================================================================================
-    // This struct collects all operations on native DependentHandles. The DependentHandle
-    // merely wraps an IntPtr so this struct serves mainly as a "managed typedef."
-    //
-    // DependentHandles exist in one of two states:
-    //
-    //    IsAllocated == false
-    //        No actual handle is allocated underneath. Illegal to call GetPrimary
-    //        or GetPrimaryAndSecondary(). Ok to call Free().
-    //
-    //        Initializing a DependentHandle using the nullary ctor creates a DependentHandle
-    //        that's in the !IsAllocated state.
-    //        (! Right now, we get this guarantee for free because (IntPtr)0 == NULL unmanaged handle.
-    //         ! If that assertion ever becomes false, we'll have to add an _isAllocated field
-    //         ! to compensate.)
-    //
-    //
-    //    IsAllocated == true
-    //        There's a handle allocated underneath. You must call Free() on this eventually
-    //        or you cause a native handle table leak.
-    //
-    // This struct intentionally does no self-synchronization. It's up to the caller to
-    // to use DependentHandles in a thread-safe way.
-    // =========================================================================================
-    internal struct DependentHandle
-    {
-        private IntPtr _handle;
-
-        public DependentHandle(object primary, object? secondary) =>
-            // no need to check for null result: nInitialize expected to throw OOM.
-            _handle = nInitialize(primary, secondary);
-
-        public bool IsAllocated => _handle != IntPtr.Zero;
-
-        // Getting the secondary object is more expensive than getting the first so
-        // we provide a separate primary-only accessor for those times we only want the
-        // primary.
-        public object? GetPrimary() => nGetPrimary(_handle);
-
-        public object? GetPrimaryAndSecondary(out object? secondary) =>
-            nGetPrimaryAndSecondary(_handle, out secondary);
-
-        public void SetPrimary(object? primary) =>
-            nSetPrimary(_handle, primary);
-
-        public void SetSecondary(object? secondary) =>
-            nSetSecondary(_handle, secondary);
-
-        // Forces dependentHandle back to non-allocated state (if not already there)
-        // and frees the handle if needed.
-        public void Free()
-        {
-            if (_handle != IntPtr.Zero)
-            {
-                IntPtr handle = _handle;
-                _handle = IntPtr.Zero;
-                nFree(handle);
-            }
-        }
-
-        [MethodImpl(MethodImplOptions.InternalCall)]
-        private static extern IntPtr nInitialize(object primary, object? secondary);
-
-        [MethodImpl(MethodImplOptions.InternalCall)]
-        private static extern object? nGetPrimary(IntPtr dependentHandle);
-
-        [MethodImpl(MethodImplOptions.InternalCall)]
-        private static extern object? nGetPrimaryAndSecondary(IntPtr dependentHandle, out object? secondary);
-
-        [MethodImpl(MethodImplOptions.InternalCall)]
-        private static extern void nSetPrimary(IntPtr dependentHandle, object? primary);
-
-        [MethodImpl(MethodImplOptions.InternalCall)]
-        private static extern void nSetSecondary(IntPtr dependentHandle, object? secondary);
-
-        [MethodImpl(MethodImplOptions.InternalCall)]
-        private static extern void nFree(IntPtr dependentHandle);
-    }
-}

src/coreclr/System.Private.CoreLib/src/System/Runtime/DependentHandle.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.CompilerServices;
+#if !DEBUG
+using Internal.Runtime.CompilerServices;
+#endif
+
+namespace System.Runtime
+{
+    /// <summary>
+    /// Represents a dependent GC handle, which will conditionally keep a dependent object instance alive as long as
+    /// a target object instance is alive as well, without representing a strong reference to the target instance.
+    /// </summary>
+    /// <remarks>
+    /// A <see cref="DependentHandle"/> value with a given object instance as target will not cause the target
+    /// to be kept alive if there are no other strong references to it, but it will do so for the dependent
+    /// object instance as long as the target is alive.
+    /// <para>
+    /// Using this type is conceptually equivalent to having a weak reference to a given target object instance A,
+    /// with that object having a field or property (or some other strong reference) to a dependent object instance B.
+    /// </para>
+    /// <para>
+    /// The <see cref="DependentHandle"/> type is not thread-safe, and consumers are responsible for ensuring that
+    /// <see cref="Dispose"/> is not called concurrently with other APIs. Not doing so results in undefined behavior.
+    /// </para>
+    /// <para>
+    /// The <see cref="IsAllocated"/>, <see cref="Target"/>, <see cref="Dependent"/> and <see cref="TargetAndDependent"/>
+    /// properties are instead thread-safe, and safe to use if <see cref="Dispose"/> is not concurrently invoked as well.
+    /// </para>
+    /// </remarks>
+    public struct DependentHandle : IDisposable
+    {
+        // =========================================================================================
+        // This struct collects all operations on native DependentHandles. The DependentHandle
+        // merely wraps an IntPtr so this struct serves mainly as a "managed typedef."
+        //
+        // DependentHandles exist in one of two states:
+        //
+        //    IsAllocated == false
+        //        No actual handle is allocated underneath. Illegal to get Target, Dependent
+        //        or GetTargetAndDependent(). Ok to call Dispose().
+        //
+        //        Initializing a DependentHandle using the nullary ctor creates a DependentHandle
+        //        that's in the !IsAllocated state.
+        //        (! Right now, we get this guarantee for free because (IntPtr)0 == NULL unmanaged handle.
+        //         ! If that assertion ever becomes false, we'll have to add an _isAllocated field
+        //         ! to compensate.)
+        //
+        //
+        //    IsAllocated == true
+        //        There's a handle allocated underneath. You must call Dispose() on this eventually
+        //        or you cause a native handle table leak.
+        //
+        // This struct intentionally does no self-synchronization. It's up to the caller to
+        // to use DependentHandles in a thread-safe way.
+        // =========================================================================================
+
+        private IntPtr _handle;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="DependentHandle"/> struct with the specified arguments.
+        /// </summary>
+        /// <param name="target">The target object instance to track.</param>
+        /// <param name="dependent">The dependent object instance to associate with <paramref name="target"/>.</param>
+        public DependentHandle(object? target, object? dependent)
+        {
+            // no need to check for null result: InternalInitialize expected to throw OOM.
+            _handle = InternalInitialize(target, dependent);
+        }
+
+        /// <summary>
+        /// Gets a value indicating whether this instance was constructed with
+        /// <see cref="DependentHandle(object?, object?)"/> and has not yet been disposed.
+        /// </summary>
+        /// <remarks>This property is thread-safe.</remarks>
+        public bool IsAllocated => (nint)_handle != 0;
+
+        /// <summary>
+        /// Gets or sets the target object instance for the current handle. The target can only be set to a <see langword="null"/> value
+        /// once the <see cref="DependentHandle"/> instance has been created. Doing so will cause <see cref="Dependent"/> to start
+        /// returning <see langword="null"/> as well, and to become eligible for collection even if the previous target is still alive.
+        /// </summary>
+        /// <exception cref="InvalidOperationException">
+        /// Thrown if <see cref="IsAllocated"/> is <see langword="false"/> or if the input value is not <see langword="null"/>.</exception>
+        /// <remarks>This property is thread-safe.</remarks>
+        public object? Target
+        {
+            get
+            {
+                IntPtr handle = _handle;
+
+                if ((nint)handle == 0)
+                {
+                    ThrowHelper.ThrowInvalidOperationException();
+                }
+
+                return InternalGetTarget(handle);
+            }
+            set
+            {
+                IntPtr handle = _handle;
+
+                if ((nint)handle == 0 || value is not null)
+                {
+                    ThrowHelper.ThrowInvalidOperationException();
+                }
+
+                InternalSetTargetToNull(handle);
+            }
+        }
+
+        /// <summary>
+        /// Gets or sets the dependent object instance for the current handle.
+        /// </summary>
+        /// <remarks>
+        /// If it is needed to retrieve both <see cref="Target"/> and <see cref="Dependent"/>, it is necessary
+        /// to ensure that the returned instance from <see cref="Target"/> will be kept alive until <see cref="Dependent"/>
+        /// is retrieved as well, or it might be collected and result in unexpected behavior. This can be done by storing the
+        /// target in a local and calling <see cref="GC.KeepAlive(object)"/> on it after <see cref="Dependent"/> is accessed.
+        /// </remarks>
+        /// <exception cref="InvalidOperationException">Thrown if <see cref="IsAllocated"/> is <see langword="false"/>.</exception>
+        /// <remarks>This property is thread-safe.</remarks>
+        public object? Dependent
+        {
+            get
+            {
+                IntPtr handle = _handle;
+
+                if ((nint)handle == 0)
+                {
+                    ThrowHelper.ThrowInvalidOperationException();
+                }
+
+                return InternalGetDependent(handle);
+            }
+            set
+            {
+                IntPtr handle = _handle;
+
+                if ((nint)handle == 0)
+                {
+                    ThrowHelper.ThrowInvalidOperationException();
+                }
+
+                InternalSetDependent(handle, value);
+            }
+        }
+
+        /// <summary>
+        /// Gets the values of both <see cref="Target"/> and <see cref="Dependent"/> (if available) as an atomic operation.
+        /// That is, even if <see cref="Target"/> is concurrently set to <see langword="null"/>, calling this method
+        /// will either return <see langword="null"/> for both target and dependent, or return both previous values.
+        /// If <see cref="Target"/> and <see cref="Dependent"/> were used sequentially in this scenario instead, it
+        /// would be possible to sometimes successfully retrieve the previous target, but then fail to get the dependent.
+        /// </summary>
+        /// <returns>The values of <see cref="Target"/> and <see cref="Dependent"/>.</returns>
+        /// <exception cref="InvalidOperationException">Thrown if <see cref="IsAllocated"/> is <see langword="false"/>.</exception>
+        /// <remarks>This property is thread-safe.</remarks>
+        public (object? Target, object? Dependent) TargetAndDependent
+        {
+            get
+            {
+                IntPtr handle = _handle;
+
+                if ((nint)handle == 0)
+                {
+                    ThrowHelper.ThrowInvalidOperationException();
+                }
+
+                object? target = InternalGetTargetAndDependent(handle, out object? dependent);
+
+                return (target, dependent);
+            }
+        }
+
+        /// <summary>
+        /// Gets the target object instance for the current handle.
+        /// </summary>
+        /// <returns>The target object instance, if present.</returns>
+        /// <remarks>This method mirrors <see cref="Target"/>, but without the allocation check.</remarks>
+        internal object? UnsafeGetTarget()
+        {
+            return InternalGetTarget(_handle);
+        }
+
+        /// <summary>
+        /// Atomically retrieves the values of both <see cref="Target"/> and <see cref="Dependent"/>, if available.
+        /// </summary>
+        /// <param name="dependent">The dependent instance, if available.</param>
+        /// <returns>The values of <see cref="Target"/> and <see cref="Dependent"/>.</returns>
+        /// <remarks>
+        /// This method mirrors the <see cref="TargetAndDependent"/> property, but without the allocation check.
+        /// The signature is also kept the same as the one for the internal call, to improve the codegen.
+        /// Note that <paramref name="dependent"/> is required to be on the stack (or it might not be tracked).
+        /// </remarks>
+        internal object? UnsafeGetTargetAndDependent(out object? dependent)
+        {
+            return InternalGetTargetAndDependent(_handle, out dependent);
+        }
+
+        /// <summary>
+        /// Sets the dependent object instance for the current handle to <see langword="null"/>.
+        /// </summary>
+        /// <remarks>This method mirrors the <see cref="Target"/> setter, but without allocation and input checks.</remarks>
+        internal void UnsafeSetTargetToNull()
+        {
+            InternalSetTargetToNull(_handle);
+        }
+
+        /// <summary>
+        /// Sets the dependent object instance for the current handle.
+        /// </summary>
+        /// <remarks>This method mirrors <see cref="Dependent"/>, but without the allocation check.</remarks>
+        internal void UnsafeSetDependent(object? dependent)
+        {
+            InternalSetDependent(_handle, dependent);
+        }
+
+        /// <inheritdoc cref="IDisposable.Dispose"/>
+        /// <remarks>This method is not thread-safe.</remarks>
+        public void Dispose()
+        {
+            // Forces the DependentHandle back to non-allocated state
+            // (if not already there) and frees the handle if needed.
+            IntPtr handle = _handle;
+
+            if ((nint)handle != 0)
+            {
+                _handle = IntPtr.Zero;
+
+                InternalFree(handle);
+            }
+        }
+
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        private static extern IntPtr InternalInitialize(object? target, object? dependent);
+
+#if DEBUG
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        private static extern object? InternalGetTarget(IntPtr dependentHandle);
+#else
+        private static unsafe object? InternalGetTarget(IntPtr dependentHandle)
+        {
+            // This optimization is the same that is used in GCHandle in RELEASE mode.
+            // This is not used in DEBUG builds as the runtime performs additional checks.
+            // The logic below is the inlined copy of ObjectFromHandle in the unmanaged runtime.
+            return Unsafe.As<IntPtr, object>(ref *(IntPtr*)(nint)dependentHandle);
+        }
+#endif
+
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        private static extern object? InternalGetDependent(IntPtr dependentHandle);
+
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        private static extern object? InternalGetTargetAndDependent(IntPtr dependentHandle, out object? dependent);
+
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        private static extern void InternalSetDependent(IntPtr dependentHandle, object? dependent);
+
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        private static extern void InternalSetTargetToNull(IntPtr dependentHandle);
+
+        [MethodImpl(MethodImplOptions.InternalCall)]
+        private static extern void InternalFree(IntPtr dependentHandle);
+    }
+}

src/coreclr/System.Private.CoreLib/src/System/Runtime/InteropServices/GCHandle.CoreCLR.cs

@@ -22,7 +22,7 @@ public partial struct GCHandle
         internal static extern object? InternalGet(IntPtr handle);
 #else
         internal static unsafe object? InternalGet(IntPtr handle) =>
-            Unsafe.As<IntPtr, object>(ref *(IntPtr*)handle);
+            Unsafe.As<IntPtr, object>(ref *(IntPtr*)(nint)handle);
 #endif
 
         [MethodImpl(MethodImplOptions.InternalCall)]

src/coreclr/vm/comdependenthandle.cpp

@@ -14,20 +14,18 @@
 #include "common.h"
 #include "comdependenthandle.h"
 
-
-
-FCIMPL2(OBJECTHANDLE, DependentHandle::nInitialize, Object *_primary, Object *_secondary)
+FCIMPL2(OBJECTHANDLE, DependentHandle::InternalInitialize, Object *_target, Object *_dependent)
 {
     FCALL_CONTRACT;
 
-    OBJECTREF primary(_primary);
-    OBJECTREF secondary(_secondary);
+    OBJECTREF target(_target);
+    OBJECTREF dependent(_dependent);
     OBJECTHANDLE result = NULL;
 
     HELPER_METHOD_FRAME_BEGIN_RET_NOPOLL();
 
     // Create the handle.
-    result = GetAppDomain()->CreateDependentHandle(primary, secondary);
+    result = GetAppDomain()->CreateDependentHandle(target, dependent);
 
     HELPER_METHOD_FRAME_END_POLL();
 
@@ -35,72 +33,80 @@ FCIMPL2(OBJECTHANDLE, DependentHandle::nInitialize, Object *_primary, Object *_s
 }
 FCIMPLEND
 
+FCIMPL1(Object*, DependentHandle::InternalGetTarget, OBJECTHANDLE handle)
+{
+    FCALL_CONTRACT;
+    FCUnique(0x54);
 
+    _ASSERTE(handle != NULL);
+
+    return OBJECTREFToObject(ObjectFromHandle(handle));
+}
+FCIMPLEND
 
-FCIMPL1(VOID, DependentHandle::nFree, OBJECTHANDLE handle)
+FCIMPL1(Object*, DependentHandle::InternalGetDependent, OBJECTHANDLE handle)
 {
     FCALL_CONTRACT;
 
     _ASSERTE(handle != NULL);
 
-    HELPER_METHOD_FRAME_BEGIN_0();
+    OBJECTREF target = ObjectFromHandle(handle);
 
-    DestroyDependentHandle(handle);
-
-    HELPER_METHOD_FRAME_END();
+    IGCHandleManager *mgr = GCHandleUtilities::GetGCHandleManager();
 
+    // The dependent is tracked only if target is non-null
+    return (target != NULL) ? mgr->GetDependentHandleSecondary(handle) : NULL;
 }
 FCIMPLEND
 
-
-
-FCIMPL1(Object*, DependentHandle::nGetPrimary, OBJECTHANDLE handle)
+FCIMPL2(Object*, DependentHandle::InternalGetTargetAndDependent, OBJECTHANDLE handle, Object **outDependent)
 {
     FCALL_CONTRACT;
-    FCUnique(0x54);
-    _ASSERTE(handle != NULL);
-    return OBJECTREFToObject(ObjectFromHandle(handle));
-}
-FCIMPLEND
 
+    _ASSERTE(handle != NULL && outDependent != NULL);
 
+    OBJECTREF target = ObjectFromHandle(handle);
+    IGCHandleManager *mgr = GCHandleUtilities::GetGCHandleManager();
+
+    // The dependent is tracked only if target is non-null
+    *outDependent = (target != NULL) ? mgr->GetDependentHandleSecondary(handle) : NULL;
+
+    return OBJECTREFToObject(target);
+}
+FCIMPLEND
 
-FCIMPL2(Object*, DependentHandle::nGetPrimaryAndSecondary, OBJECTHANDLE handle, Object **outSecondary)
+FCIMPL1(VOID, DependentHandle::InternalSetTargetToNull, OBJECTHANDLE handle)
 {
     FCALL_CONTRACT;
-    _ASSERTE(handle != NULL && outSecondary != NULL);
 
-    OBJECTREF primary = ObjectFromHandle(handle);
+    _ASSERTE(handle != NULL);
 
     IGCHandleManager *mgr = GCHandleUtilities::GetGCHandleManager();
-    // Secondary is tracked only if primary is non-null
-    *outSecondary = (primary != NULL) ? mgr->GetDependentHandleSecondary(handle) : NULL;
-
-    return OBJECTREFToObject(primary);
+    mgr->StoreObjectInHandle(handle, NULL);
 }
 FCIMPLEND
 
-FCIMPL2(VOID, DependentHandle::nSetPrimary, OBJECTHANDLE handle, Object *_primary)
+FCIMPL2(VOID, DependentHandle::InternalSetDependent, OBJECTHANDLE handle, Object *_dependent)
 {
     FCALL_CONTRACT;
 
     _ASSERTE(handle != NULL);
 
-    // Avoid collision with MarshalNative::GCHandleInternalSet
-    FCUnique(0x12);
-
     IGCHandleManager *mgr = GCHandleUtilities::GetGCHandleManager();
-    mgr->StoreObjectInHandle(handle, _primary);
+    mgr->SetDependentHandleSecondary(handle, _dependent);
 }
 FCIMPLEND
 
-FCIMPL2(VOID, DependentHandle::nSetSecondary, OBJECTHANDLE handle, Object *_secondary)
+FCIMPL1(VOID, DependentHandle::InternalFree, OBJECTHANDLE handle)
 {
     FCALL_CONTRACT;
 
     _ASSERTE(handle != NULL);
 
-    IGCHandleManager *mgr = GCHandleUtilities::GetGCHandleManager();
-    mgr->SetDependentHandleSecondary(handle, _secondary);
+    HELPER_METHOD_FRAME_BEGIN_0();
+
+    DestroyDependentHandle(handle);
+
+    HELPER_METHOD_FRAME_END();
 }
 FCIMPLEND

src/coreclr/vm/comdependenthandle.h

@@ -16,16 +16,16 @@
 
 // A dependent handle is conceputally a tuple containing two object reference
 //
-//     * A Primary object (think key)
-//     * A Secondary Object (think value)
+//     * A Target object (think key)
+//     * A Dependent Object (think value)
 //
-// The reference to both the primary object is (long) weak (will not keep the object alive). However the
-// reference to the secondary object is (long) weak if the primary object is dead, and strong if the primary
-// object is alive. (Hence it is a 'Dependent' handle since the strength of the secondary reference depends
-// on the primary).
+// The reference to both the target object is (long) weak (will not keep the object alive). However the
+// reference to the dependent object is (long) weak if the target object is dead, and strong if the target
+// object is alive. (Hence it is a 'Dependent' handle since the strength of the dependent reference depends
+// on the target).
 //
 // The effect of this semantics is that it seems that while the DependentHandle exists, the system behaves as
-// if there was a normal strong reference from the primary object to the secondary one.
+// if there was a normal strong reference from the target object to the dependent one.
 //
 // The usefulness of a DependentHandle is to allow other objects to be 'attached' to a given object. By
 // having a hash table where the entries are dependent handles you can attach arbitrary objects to another
@@ -40,13 +40,13 @@
 class DependentHandle
 {
 public:
-    static FCDECL2(OBJECTHANDLE, nInitialize, Object *primary, Object *secondary);
-    static FCDECL1(Object *, nGetPrimary, OBJECTHANDLE handle);
-    static FCDECL2(Object *, nGetPrimaryAndSecondary, OBJECTHANDLE handle, Object **outSecondary);
-    static FCDECL1(VOID, nFree, OBJECTHANDLE handle);
-    static FCDECL2(VOID, nSetPrimary, OBJECTHANDLE handle, Object *primary);
-    static FCDECL2(VOID, nSetSecondary, OBJECTHANDLE handle, Object *secondary);
+    static FCDECL2(OBJECTHANDLE, InternalInitialize, Object *target, Object *dependent);
+    static FCDECL1(Object *, InternalGetTarget, OBJECTHANDLE handle);
+    static FCDECL1(Object *, InternalGetDependent, OBJECTHANDLE handle);
+    static FCDECL2(Object *, InternalGetTargetAndDependent, OBJECTHANDLE handle, Object **outDependent);
+    static FCDECL1(VOID, InternalSetTargetToNull, OBJECTHANDLE handle);
+    static FCDECL2(VOID, InternalSetDependent, OBJECTHANDLE handle, Object *dependent);
+    static FCDECL1(VOID, InternalFree, OBJECTHANDLE handle);
 };
 
 #endif
-

src/coreclr/vm/ecalllist.h

@@ -61,12 +61,13 @@
 
 
 FCFuncStart(gDependentHandleFuncs)
-    FCFuncElement("nInitialize",             DependentHandle::nInitialize)
-    FCFuncElement("nGetPrimary",             DependentHandle::nGetPrimary)
-    FCFuncElement("nGetPrimaryAndSecondary", DependentHandle::nGetPrimaryAndSecondary)
-    FCFuncElement("nFree",                   DependentHandle::nFree)
-    FCFuncElement("nSetPrimary",             DependentHandle::nSetPrimary)
-    FCFuncElement("nSetSecondary",           DependentHandle::nSetSecondary)
+    FCFuncElement("InternalInitialize",            DependentHandle::InternalInitialize)
+    FCFuncElement("InternalGetTarget",             DependentHandle::InternalGetTarget)
+    FCFuncElement("InternalGetDependent",          DependentHandle::InternalGetDependent)
+    FCFuncElement("InternalGetTargetAndDependent", DependentHandle::InternalGetTargetAndDependent)
+    FCFuncElement("InternalSetTargetToNull",       DependentHandle::InternalSetTargetToNull)
+    FCFuncElement("InternalSetDependent",          DependentHandle::InternalSetDependent)
+    FCFuncElement("InternalFree",                  DependentHandle::InternalFree)
 FCFuncEnd()
 
 
@@ -1139,7 +1140,7 @@ FCClassElement("CustomAttribute", "System.Reflection", gCOMCustomAttributeFuncs)
 FCClassElement("CustomAttributeEncodedArgument", "System.Reflection", gCustomAttributeEncodedArgument)
 FCClassElement("Debugger", "System.Diagnostics", gDiagnosticsDebugger)
 FCClassElement("Delegate", "System", gDelegateFuncs)
-FCClassElement("DependentHandle", "System.Runtime.CompilerServices", gDependentHandleFuncs)
+FCClassElement("DependentHandle", "System.Runtime", gDependentHandleFuncs)
 FCClassElement("Enum", "System", gEnumFuncs)
 FCClassElement("Environment", "System", gEnvironmentFuncs)
 #if defined(FEATURE_PERFTRACING)

src/libraries/System.Private.CoreLib/src/System/Runtime/CompilerServices/ConditionalWeakTable.cs

@@ -515,6 +515,7 @@ internal bool TryGetValueWorker(TKey key, [MaybeNullWhen(false)] out TValue valu
             /// Returns -1 if not found (if key expires during FindEntry, this can be treated as "not found.").
             /// Must hold _lock, or be prepared to retry the search while holding _lock.
             /// </summary>
+            /// <remarks>This method requires <paramref name="value"/> to be on the stack to be properly tracked.</remarks>
             internal int FindEntry(TKey key, out object? value)
             {
                 Debug.Assert(key != null); // Key already validated as non-null.
@@ -523,14 +524,15 @@ internal int FindEntry(TKey key, out object? value)
                 int bucket = hashCode & (_buckets.Length - 1);
                 for (int entriesIndex = Volatile.Read(ref _buckets[bucket]); entriesIndex != -1; entriesIndex = _entries[entriesIndex].Next)
                 {
-                    if (_entries[entriesIndex].HashCode == hashCode && _entries[entriesIndex].depHnd.GetPrimaryAndSecondary(out value) == key)
+                    if (_entries[entriesIndex].HashCode == hashCode && _entries[entriesIndex].depHnd.UnsafeGetTargetAndDependent(out value) == key)
                     {
-                        GC.KeepAlive(this); // ensure we don't get finalized while accessing DependentHandles.
+                        GC.KeepAlive(this); // Ensure we don't get finalized while accessing DependentHandle
+
                         return entriesIndex;
                     }
                 }
 
-                GC.KeepAlive(this); // ensure we don't get finalized while accessing DependentHandles.
+                GC.KeepAlive(this); // Ensure we don't get finalized while accessing DependentHandle
                 value = null;
                 return -1;
             }
@@ -540,8 +542,9 @@ internal bool TryGetEntry(int index, [NotNullWhen(true)] out TKey? key, [MaybeNu
             {
                 if (index < _entries.Length)
                 {
-                    object? oKey = _entries[index].depHnd.GetPrimaryAndSecondary(out object? oValue);
-                    GC.KeepAlive(this); // ensure we don't get finalized while accessing DependentHandles.
+                    object? oKey = _entries[index].depHnd.UnsafeGetTargetAndDependent(out object? oValue);
+
+                    GC.KeepAlive(this); // Ensure we don't get finalized while accessing DependentHandle
 
                     if (oKey != null)
                     {
@@ -592,7 +595,7 @@ private void RemoveIndex(int entryIndex)
                 Volatile.Write(ref entry.HashCode, -1);
 
                 // Also, clear the key to allow GC to collect objects pointed to by the entry
-                entry.depHnd.SetPrimary(null);
+                entry.depHnd.UnsafeSetTargetToNull();
             }
 
             internal void UpdateValue(int entryIndex, TValue newValue)
@@ -602,7 +605,7 @@ internal void UpdateValue(int entryIndex, TValue newValue)
                 VerifyIntegrity();
                 _invalid = true;
 
-                _entries[entryIndex].depHnd.SetSecondary(newValue);
+                _entries[entryIndex].depHnd.UnsafeSetDependent(newValue);
 
                 _invalid = false;
             }
@@ -634,7 +637,7 @@ internal Container Resize()
                             break;
                         }
 
-                        if (entry.depHnd.IsAllocated && entry.depHnd.GetPrimary() is null)
+                        if (entry.depHnd.IsAllocated && entry.depHnd.UnsafeGetTarget() is null)
                         {
                             // the entry has expired
                             hasExpiredEntries = true;
@@ -699,7 +702,7 @@ internal Container Resize(int newSize)
                         DependentHandle depHnd = oldEntry.depHnd;
                         if (hashCode != -1 && depHnd.IsAllocated)
                         {
-                            if (depHnd.GetPrimary() != null)
+                            if (depHnd.UnsafeGetTarget() is not null)
                             {
                                 ref Entry newEntry = ref newEntries[newEntriesIndex];
 
@@ -795,7 +798,7 @@ private void VerifyIntegrity()
                         //   another container, removed entries are not, therefore this container must free them.
                         if (_oldKeepAlive is null || entries[entriesIndex].HashCode == -1)
                         {
-                            entries[entriesIndex].depHnd.Free();
+                            entries[entriesIndex].depHnd.Dispose();
                         }
                     }
                 }

src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/GCHandle.cs

@@ -142,7 +142,7 @@ public IntPtr AddrOfPinnedObject()
         }
 
         /// <summary>Determine whether this handle has been allocated or not.</summary>
-        public bool IsAllocated => _handle != IntPtr.Zero;
+        public bool IsAllocated => (nint)_handle != 0;
 
         /// <summary>
         /// Used to create a GCHandle from an int.  This is intended to
@@ -165,9 +165,9 @@ public static GCHandle FromIntPtr(IntPtr value)
 
         public override bool Equals([NotNullWhen(true)] object? o) => o is GCHandle && _handle == ((GCHandle)o)._handle;
 
-        public static bool operator ==(GCHandle a, GCHandle b) => a._handle == b._handle;
+        public static bool operator ==(GCHandle a, GCHandle b) => (nint)a._handle == (nint)b._handle;
 
-        public static bool operator !=(GCHandle a, GCHandle b) => a._handle != b._handle;
+        public static bool operator !=(GCHandle a, GCHandle b) => (nint)a._handle != (nint)b._handle;
 
         [MethodImpl(MethodImplOptions.AggressiveInlining)]
         private static IntPtr GetHandleValue(IntPtr handle) => new IntPtr((nint)handle & ~(nint)1); // Remove Pin flag
@@ -179,7 +179,7 @@ public static GCHandle FromIntPtr(IntPtr value)
         private static void ThrowIfInvalid(IntPtr handle)
         {
             // Check if the handle was never initialized or was freed.
-            if (handle == IntPtr.Zero)
+            if ((nint)handle == 0)
             {
                 ThrowHelper.ThrowInvalidOperationException_HandleIsNotInitialized();
             }

src/libraries/System.Runtime/ref/System.Runtime.cs

@@ -9593,6 +9593,17 @@ public sealed partial class AssemblyTargetedPatchBandAttribute : System.Attribut
         public AssemblyTargetedPatchBandAttribute(string targetedPatchBand) { }
         public string TargetedPatchBand { get { throw null; } }
     }
+    public partial struct DependentHandle : System.IDisposable
+    {
+        private object _dummy;
+        private int _dummyPrimitive;
+        public DependentHandle(object? target, object? dependent) { throw null; }
+        public object? Dependent { get { throw null; } set { } }
+        public bool IsAllocated { get { throw null; } }
+        public object? Target { get { throw null; } set { } }
+        public (object? Target, object? Dependent) TargetAndDependent { get { throw null; } }
+        public void Dispose() { }
+    }
     public enum GCLargeObjectHeapCompactionMode
     {
         Default = 1,

src/libraries/System.Runtime/tests/System.Runtime.Tests.csproj

@@ -215,6 +215,7 @@
     <Compile Include="System\Reflection\TypeDelegatorTests.cs" />
     <Compile Include="System\Reflection\TypeTests.Get.CornerCases.cs" />
     <Compile Include="System\Reflection\TypeTests.GetMember.cs" />
+    <Compile Include="System\Runtime\DependentHandleTests.cs" />
     <Compile Include="System\Runtime\MemoryFailPointTests.cs" />
     <Compile Include="System\Runtime\NgenServicingAttributesTests.cs" />
     <Compile Include="System\Runtime\CompilerServices\AttributesTests.cs" />

src/libraries/System.Runtime/tests/System/Runtime/DependentHandleTests.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.CompilerServices;
+using Xunit;
+
+namespace System.Runtime.Tests
+{
+    // NOTE: DependentHandle is already heavily tested indirectly through ConditionalWeakTable<,>.
+    // This class contains some specific tests for APIs that are only relevant when used directly.
+    public class DependentHandleTests
+    {
+        [Fact]
+        public void GetNullTarget()
+        {
+            object target = new();
+            DependentHandle handle = new(null, null);
+
+            Assert.True(handle.IsAllocated);
+            Assert.Null(handle.Target);
+            Assert.Null(handle.Dependent);
+
+            handle.Dispose();
+        }
+
+        [Fact]
+        public void GetNotNullTarget()
+        {
+            object target = new();
+            DependentHandle handle = new(target, null);
+
+            // A handle with a set target and no dependent is valid
+            Assert.True(handle.IsAllocated);
+            Assert.Same(target, handle.Target);
+            Assert.Null(handle.Dependent);
+
+            handle.Dispose();
+        }
+
+        [Fact]
+        public void SetTargetToNull_StateIsConsistent()
+        {
+            object target = new(), dependent = new();
+            DependentHandle handle = new(target, dependent);
+
+            Assert.True(handle.IsAllocated);
+            Assert.Same(handle.Target, target);
+            Assert.Same(handle.Dependent, dependent);
+
+            handle.Target = null;
+
+            Assert.True(handle.IsAllocated);
+            Assert.Null(handle.Target);
+            Assert.Null(handle.Dependent);
+
+            handle.Dispose();
+        }
+
+        [Fact]
+        public void SetTargetToNull_RepeatedCallsAreFine()
+        {
+            object target = new(), dependent = new();
+            DependentHandle handle = new(target, dependent);
+
+            handle.Target = null;
+
+            Assert.True(handle.IsAllocated);
+            Assert.Null(handle.Target);
+            Assert.Null(handle.Dependent);
+
+            handle.Target = null;
+            handle.Target = null;
+            handle.Target = null;
+
+            Assert.True(handle.IsAllocated);
+            Assert.Null(handle.Target);
+            Assert.Null(handle.Dependent);
+
+            handle.Dispose();
+        }
+
+        [Fact]
+        public void GetSetDependent()
+        {
+            object target = new(), dependent = new();
+            DependentHandle handle = new(target, null);
+
+            // The target can be retrieved correctly
+            Assert.True(handle.IsAllocated);
+            Assert.Same(target, handle.Target);
+            Assert.Null(handle.Dependent);
+
+            handle.Dependent = dependent;
+
+            // The dependent can also be retrieved correctly
+            Assert.Same(target, handle.Target);
+            Assert.Same(dependent, handle.Dependent);
+
+            handle.Dispose();
+        }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsPreciseGcSupported))]
+        public void TargetKeepsDependentAlive()
+        {
+            [MethodImpl(MethodImplOptions.NoInlining)]
+            static DependentHandle Initialize(out object target, out WeakReference weakDependent)
+            {
+                target = new object();
+
+                object dependent = new();
+
+                weakDependent = new WeakReference(dependent);
+
+                return new DependentHandle(target, dependent);
+            }
+
+            DependentHandle handle = Initialize(out object target, out WeakReference dependent);
+
+            GC.Collect();
+
+            // The dependent has to still be alive as the target has a strong reference
+            Assert.Same(target, handle.Target);
+            Assert.True(dependent.IsAlive);
+            Assert.Same(dependent.Target, handle.Dependent);
+
+            GC.KeepAlive(target);
+
+            handle.Dispose();
+        }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsPreciseGcSupported))]
+        public void DependentDoesNotKeepTargetAlive()
+        {
+            [MethodImpl(MethodImplOptions.NoInlining)]
+            static DependentHandle Initialize(out WeakReference weakTarget, out object dependent)
+            {
+                dependent = new object();
+
+                object target = new();
+
+                weakTarget = new WeakReference(target);
+
+                return new DependentHandle(target, dependent);
+            }
+
+            DependentHandle handle = Initialize(out WeakReference target, out object dependent);
+
+            GC.Collect();
+
+            // The target has to be collected, as there were no strong references to it
+            Assert.Null(handle.Target);
+            Assert.False(target.IsAlive);
+
+            GC.KeepAlive(target);
+
+            handle.Dispose();
+        }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsPreciseGcSupported))]
+        public void DependentIsCollectedOnTargetNotReachable()
+        {
+            [MethodImpl(MethodImplOptions.NoInlining)]
+            static DependentHandle Initialize(out WeakReference weakTarget, out WeakReference weakDependent)
+            {
+                object target = new(), dependent = new();
+
+                weakTarget = new WeakReference(target);
+                weakDependent = new WeakReference(dependent);
+
+                return new DependentHandle(target, dependent);
+            }
+
+            DependentHandle handle = Initialize(out WeakReference target, out WeakReference dependent);
+
+            GC.Collect();
+
+            // Both target and dependent have to be collected, as there were no strong references to either
+            Assert.Null(handle.Target);
+            Assert.Null(handle.Dependent);
+            Assert.False(target.IsAlive);
+            Assert.False(dependent.IsAlive);
+
+            handle.Dispose();
+        }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsPreciseGcSupported))]
+        public void DependentIsCollectedOnTargetNotReachable_EvenWithReferenceCycles()
+        {
+            [MethodImpl(MethodImplOptions.NoInlining)]
+            static DependentHandle Initialize(out WeakReference weakTarget, out WeakReference weakDependent)
+            {
+                object target = new();
+                ObjectWithReference dependent = new() { Reference = target };
+
+                weakTarget = new WeakReference(target);
+                weakDependent = new WeakReference(dependent);
+
+                return new DependentHandle(target, dependent);
+            }
+
+            DependentHandle handle = Initialize(out WeakReference target, out WeakReference dependent);
+
+            GC.Collect();
+
+            // Both target and dependent have to be collected, as there were no strong references to either.
+            // The fact that the dependent has a strong reference back to the target should not affect this.
+            Assert.Null(handle.Target);
+            Assert.Null(handle.Dependent);
+            Assert.False(target.IsAlive);
+            Assert.False(dependent.IsAlive);
+
+            handle.Dispose();
+        }
+
+        private sealed class ObjectWithReference
+        {
+            public object Reference;
+        }
+
+        [ConditionalFact(typeof(PlatformDetection), nameof(PlatformDetection.IsPreciseGcSupported))]
+        public void DependentIsCollectedAfterTargetIsSetToNull()
+        {
+            [MethodImpl(MethodImplOptions.NoInlining)]
+            static DependentHandle Initialize(out object target, out WeakReference weakDependent)
+            {
+                target = new();
+
+                object dependent = new();
+
+                weakDependent = new WeakReference(dependent);
+
+                return new DependentHandle(target, dependent);
+            }
+
+            DependentHandle handle = Initialize(out object target, out WeakReference dependent);
+
+            handle.Target = null;
+
+            GC.Collect();
+
+            // After calling StopTracking, the dependent is collected even if
+            // target is still alive and the handle itself has not been disposed
+            Assert.True(handle.IsAllocated);
+            Assert.Null(handle.Target);
+            Assert.Null(handle.Dependent);
+            Assert.False(dependent.IsAlive);
+
+            GC.KeepAlive(target);
+
+            handle.Dispose();
+        }
+
+        [Fact]
+        public void GetTarget_ThrowsInvalidOperationException()
+        {
+            Assert.Throws<InvalidOperationException>(() => default(DependentHandle).Target);
+        }
+
+        [Fact]
+        public void GetDependent_ThrowsInvalidOperationException()
+        {
+            Assert.Throws<InvalidOperationException>(() => default(DependentHandle).Dependent);
+        }
+
+        [Fact]
+        public void SetTarget_NotAllocated_ThrowsInvalidOperationException()
+        {
+            Assert.Throws<InvalidOperationException>(() =>
+            {
+                DependentHandle handle = default;
+                handle.Target = new();
+            });
+        }
+
+        [Fact]
+        public void SetTarget_NotNullObject_ThrowsInvalidOperationException()
+        {
+            Assert.Throws<InvalidOperationException>(() =>
+            {
+                DependentHandle handle = default;
+
+                try
+                {
+                    handle.Target = new();
+                }
+                finally
+                {
+                    handle.Dispose();
+                }                
+            });
+        }
+
+        [Fact]
+        public void SetDependent_ThrowsInvalidOperationException()
+        {
+            Assert.Throws<InvalidOperationException>(() =>
+            {
+                DependentHandle handle = default;
+                handle.Dependent = new();
+            });
+        }
+
+        [Fact]
+        public void Dispose_RepeatedCallsAreFine()
+        {
+            object target = new(), dependent = new();
+            DependentHandle handle = new(target, dependent);
+
+            Assert.True(handle.IsAllocated);
+
+            handle.Dispose();
+
+            Assert.False(handle.IsAllocated);
+
+            handle.Dispose();
+
+            Assert.False(handle.IsAllocated);
+
+            handle.Dispose();
+            handle.Dispose();
+            handle.Dispose();
+
+            Assert.False(handle.IsAllocated);
+        }
+
+        [Fact]
+        public void Dispose_ValidOnDefault()
+        {
+            DependentHandle handle = default;
+            Assert.False(handle.IsAllocated);
+            handle.Dispose();
+        }
+    }
+}

src/mono/System.Private.CoreLib/System.Private.CoreLib.csproj

@@ -241,8 +241,8 @@
       <Compile Include="$(BclSourcesRoot)\System\Reflection\Metadata\AssemblyExtensions.cs" />
       <Compile Include="$(BclSourcesRoot)\System\Reflection\Metadata\MetadataUpdater.cs" />
       <Compile Include="$(BclSourcesRoot)\System\Resources\ManifestBasedResourceGroveler.Mono.cs" />
+      <Compile Include="$(BclSourcesRoot)\System\Runtime\DependentHandle.cs" />
       <Compile Include="$(BclSourcesRoot)\System\Runtime\GCSettings.Mono.cs" />
-      <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\DependentHandle.cs" />
       <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\JitHelpers.cs" />
       <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\RuntimeHelpers.Mono.cs" />
       <Compile Include="$(BclSourcesRoot)\System\Runtime\CompilerServices\RuntimeFeature.Mono.cs" />

src/mono/System.Private.CoreLib/src/System/GC.Mono.cs

 // Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
+using System.Runtime;
 using System.Runtime.CompilerServices;
 using Internal.Runtime.CompilerServices;
 using System.Diagnostics.Tracing;

src/mono/System.Private.CoreLib/src/System/Runtime/CompilerServices/DependentHandle.cs

-// Licensed to the .NET Foundation under one or more agreements.
-// The .NET Foundation licenses this file to you under the MIT license.
-
-namespace System.Runtime.CompilerServices
-{
-    internal struct Ephemeron
-    {
-        public object? key;
-        public object? value;
-    }
-
-    //
-    // Instead of dependent handles, mono uses arrays of Ephemeron objects.
-    //
-    internal struct DependentHandle
-    {
-        private Ephemeron[] data;
-
-        public DependentHandle(object? primary, object? secondary)
-        {
-            data = new Ephemeron[1];
-            data[0].key = primary;
-            data[0].value = secondary;
-            GC.register_ephemeron_array(data);
-        }
-
-        public bool IsAllocated => data != null;
-
-        // Getting the secondary object is more expensive than getting the first so
-        // we provide a separate primary-only accessor for those times we only want the
-        // primary.
-        public object? GetPrimary()
-        {
-            if (!IsAllocated)
-                throw new NotSupportedException();
-            if (data[0].key == GC.EPHEMERON_TOMBSTONE)
-                return null;
-            return data[0].key;
-        }
-
-        public object? GetPrimaryAndSecondary(out object? secondary)
-        {
-            if (!IsAllocated)
-                throw new NotSupportedException();
-            if (data[0].key == GC.EPHEMERON_TOMBSTONE)
-            {
-                secondary = null;
-                return null;
-            }
-            secondary = data[0].value;
-            return data[0].key;
-        }
-
-        public void SetPrimary(object? primary)
-        {
-            if (!IsAllocated)
-                throw new NotSupportedException();
-            data[0].key = primary;
-        }
-
-        public void SetSecondary(object? secondary)
-        {
-            if (!IsAllocated)
-                throw new NotSupportedException();
-            data[0].value = secondary;
-        }
-
-        public void Free()
-        {
-            data = null!;
-        }
-    }
-}

src/mono/System.Private.CoreLib/src/System/Runtime/DependentHandle.cs

+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.Runtime
+{
+    internal struct Ephemeron
+    {
+        public object? Key;
+        public object? Value;
+    }
+
+    //
+    // Instead of dependent handles, mono uses arrays of Ephemeron objects.
+    //
+    public struct DependentHandle : IDisposable
+    {
+        private Ephemeron[]? _data;
+
+        public DependentHandle(object? target, object? dependent)
+        {
+            _data = new Ephemeron[1];
+            _data[0].Key = target;
+            _data[0].Value = dependent;
+            GC.register_ephemeron_array(_data);
+        }
+
+        public bool IsAllocated => _data is not null;
+
+        public object? Target
+        {
+            get => UnsafeGetTarget();
+            set
+            {
+                Ephemeron[]? data = _data;
+
+                if (data is null || value is not null)
+                {
+                    ThrowHelper.ThrowInvalidOperationException();
+
+                    return;
+                }
+
+                data[0].Key = null;
+            }
+        }
+
+        public object? Dependent
+        {
+            get => UnsafeGetDependent();
+            set => UnsafeSetDependent(value);
+        }
+
+        public (object? Target, object? Dependent) TargetAndDependent
+        {
+            get
+            {
+                object? target = UnsafeGetTargetAndDependent(out object? dependent);
+
+                return (target, dependent);
+            }
+        }
+
+        internal object? UnsafeGetTarget()
+        {
+            Ephemeron[]? data = _data;
+
+            if (data is null)
+            {
+                ThrowHelper.ThrowInvalidOperationException();
+
+                return default;
+            }
+
+            object? key = data[0].Key;
+
+            return key != GC.EPHEMERON_TOMBSTONE ? key : null;
+        }
+
+        internal object? UnsafeGetDependent()
+        {
+            Ephemeron[]? data = _data;
+
+            if (data is null)
+            {
+                ThrowHelper.ThrowInvalidOperationException();
+
+                return default;
+            }
+
+            Ephemeron e = data[0];
+
+            return e.Key != GC.EPHEMERON_TOMBSTONE && e.Key is not null ? e.Value : null;
+        }
+
+        internal object? UnsafeGetTargetAndDependent(out object? dependent)
+        {
+            Ephemeron[]? data = _data;
+
+            if (data is null)
+            {
+                ThrowHelper.ThrowInvalidOperationException();
+
+                dependent = null;
+
+                return null;
+            }
+
+            Ephemeron e = data[0];
+
+            if (e.Key != GC.EPHEMERON_TOMBSTONE && e.Key is not null)
+            {
+                dependent = e.Value;
+
+                return e.Key;
+            }
+
+            dependent = null;
+
+            return null;
+        }
+
+        internal void UnsafeSetTargetToNull()
+        {
+            Ephemeron[]? data = _data;
+
+            if (data is null)
+            {
+                ThrowHelper.ThrowInvalidOperationException();
+
+                return;
+            }
+
+            data[0].Key = null;
+        }
+
+        internal void UnsafeSetDependent(object? dependent)
+        {
+            Ephemeron[]? data = _data;
+
+            if (data is null)
+            {
+                ThrowHelper.ThrowInvalidOperationException();
+
+                return;
+            }
+
+            data[0].Value = dependent;
+        }
+
+        public void Dispose()
+        {
+            _data = null;
+        }
+    }
+}